Such validators generally comprise a sensing system for generating two or more measurement signals, and a processing system for determining acceptability based on the signals and on stored data defining acceptability criteria corresponding to a valid type of article.
The acceptability criteria generally define an area or volume (in a measurement space defined by axes corresponding to the measurement signals) determined by, and encompassing, the statistical distribution of measurements from a population of known genuine articles.
The distributions of genuine articles may overlap with those of others, or with those of forgeries, counterfeits or slugs.
GB-A-2272319 discloses a coin validator using an acceptance region with a curved boundary.
EP-A-0367921, EP-A-0505609, U.S. Pat. No. 5,351,798 and WO-A-92/18951 disclose coin validators using acceptance regions having an ellipsoidal or circular boundary.
WO-A-92/18951, GB-A-2251111 and U.S. Pat. No. 5,351,798 disclose coin validators in which a coin is classified as one of several types in dependence upon the Mahalanobis distance (i.e. the square of the Euclidean distance in a space in which the measurements are each normalised by the variance) from the coin measurement to the center of the distribution of each type.
EP-A-0560023 discloses a banknote validator in which a banknote is accepted as genuine if its measurements define a point within a predetermined Mahalanobis distance from the center of a valid banknote distribution.
An acceptance region boundary defined by a fixed Mahalanobis distance corresponds to an ellipsoidal boundary, and also defines a contour of equal probability (assuming the distribution of genuine coins is unimodal and Normal (Gaussian)) that measurements of a genuine coin are likely to fall within the boundary.
The above-described systems may represent an advance in many areas of validation. Their operation is, however, predicated on the unspoken assumption that the probability of given measurements being associated with a particular item type (the a posteriori probability) is well correlated with the probability of an item of that type exhibiting those measurements.